This invention generally relates to intravascular balloon catheters, such as are used in percutaneous transluminal coronary angioplasty (PTCA) and stent delivery.
PTCA is a widely used procedure for the treatment of coronary heart disease. In this procedure, a balloon dilatation catheter is advanced into the patient""s coronary artery and the balloon on the catheter is inflated within the stenotic region of the patient""s artery to open up the arterial passageway and thereby increase the blood flow there through. To facilitate the advancement of the dilatation catheter into the patient""s coronary artery, a guiding catheter having a preshaped distal tip is first percutaneously introduced into the cardiovascular system of a patient by the Seldinger technique through the brachial or femoral arteries. The catheter is advanced until the preshaped distal tip of the guiding catheter is disposed within the aorta adjacent the ostium of the desired coronary artery, and the distal tip of the guiding catheter is then maneuvered into the ostium. A balloon dilatation catheter may then be advanced through the guiding catheter into the patient""s coronary artery over a guidewire until the balloon on the catheter is disposed within the stenotic region of the patient""s artery.
The balloon is inflated to open up the arterial passageway and increase the blood flow through the artery. Generally, the inflated diameter of the balloon is approximately the same diameter as the native diameter of the body lumen being dilated so as to complete the dilatation but not over expand the artery wall. After the balloon is finally deflated, blood flow resumes through the dilated artery and the dilatation catheter can be removed therefrom.
In a large number of angioplasty procedures, there may be a restenosis, i.e. reformation of the arterial plaque. To reduce the restenosis rate and to strengthen the dilated area, physicians now frequently implant an intravascular prosthesis called a stent inside the artery at the site of the lesion. Stents may also be used to repair vessels having an intimal flap or dissection or to generally strengthen a weakened section of a vessel. Stents are usually delivered to a desired location within a coronary artery in a contracted condition on a balloon of a catheter which is similar in many respects to a balloon angioplasty catheter, and expanded to a larger diameter by expansion of the balloon. The balloon is deflated to remove the catheter and the stent is left in place within the artery at the site of the dilated lesion.
Catheter balloons are typically manufactured independently of the catheter and then secured to the catheter with an adhesive or other bonding method. In standard balloon manufacture, an extruded polymer tube is blown biaxially under the action of axial tension, internal pressure and heat into a mold. The polymeric tube has an initial outer diameter, and is elongated axially until the polymeric tube exhibits resistance to further stretching. This is generally achieved at about 2 to 4 times the original length, the outer diameter is roughly 40% to 50% of the original outer diameter, and the wall thickness is about 50% of the original wall thickness. The blowing temperature is generally above the glass transition temperature of the polymer, but below its melting point. The pressurization fluids may include air, nitrogen or argon. Additionally, pressurized liquids such as water or alcohol have been used. The polymer tube may either be simultaneously stretched in the radial and axial directions, or sequentially, by first stretching axially and then radially.
The balloon blow-up-ratio (BUR) is defined as the ratio of the outer diameter of the blown balloon to the inner diameter of the extruded tubing. For a given balloon wall thickness, the rupture strength generally increases and the radial compliance decreases as the balloon BUR increases. This is due to the increase in the molecular orientation of the polymer with increased stretching. For standard pressure driven blow molding, typical BURs range from about 4.5 to about 7.0 depending on the material and the product application. Beyond a critical BUR for a given polymer, the balloon blowing process becomes unstable and the tubing often ruptures before a balloon is formed. For some materials, defects such as microtears appear above a certain BUR because of the combination of the high rate and high extent of material stretching.
In the standard pressurized blow molding process, an initiated bubble rapidly grows in diameter until it is constrained by the mold wall. The hoop stress in the wall of the tubing, as it grows into a balloon may be approximated by the expression:       σ    h    =            P      ·      R        δ  
where P is the inflation pressure, R is the mean radius of the polymeric tube at any time during the inflation and xcex4 is the wall thickness of the tubing. For a balloon to be initiated from the tubing, the inflation pressure should be such that the wall hoop stress exceeds the material resistance (typically the yield stress) to stretching at the blowing temperature. Once a balloon is initiated from the tubing, it grows rapidly in size until it touches the mold wall. As the balloon grows, the radius increases and the balloon wall thickness decreases. This results in a rapid increase in the wall hoop stress during constant pressure blowing. If the wall hoop stress of the growing balloon exceeds the ultimate hoop strength of the material, rupture will occur. The situation is exacerbated for high BURs. It would be desirable to reduce the blowing pressure as the balloon grows, but it is difficult to modulate pressure in the rapid time scale of tubing growth into a balloon. What has been needed is a method of blowing balloons that will allow greater control of the process.
The present invention is directed to a method of forming a balloon for a catheter. The method generally includes providing a polymeric tubular member having an inner lumen and a longitudinal axis, sealing off one end of the tubular member, introducing an incompressible fluid into the lumen at a volumetric flow rate, and expanding the polymeric tubular member to a desired outer diameter. The invention is also directed to a balloon formed by the method of the invention in which the polymeric tubular member is expanded to form the balloon by volumetric metering of an incompressible fluid into the lumen of the polymeric tubular member. In a presently preferred embodiment, the balloon is formed of a co-polyester such as Hytrel available from E.I. DuPont de Nemours, or Arnitel available from DSM Engineering, and in one embodiment, a polyester block copolymer having a polybutylene terephthalate hard segment. In a presently preferred embodiment, the balloon blow up ratio is about 5 to about 8. In one embodiment, the balloon blow up ratio is at least about 6.
The polymeric tubular member is typically radially expanded at elevated temperature. The temperature of the tubular member during expansion is generally higher than the glass transition temperature of the polymer, but lower than the melting point. In one embodiment, axial tension is applied to the polymeric tubular member to lengthen the polymeric tubular member during formation of the balloon. The axial tension may be applied before the infusion of the incompressible fluid.
Introduction of an incompressible fluid into the tubular member will cause an increase in the lumen volume by an amount equal to the amount of fluid infused into the lumen. The rate of fluid infusion may be slowed once a balloon is initiated upon material yielding, in order to stabilize the growing balloon. Because of the volumetric control of the infusion, the initiated radially expanding section, or bubble, or the balloon does not grow in volume beyond the volume of fluid infused. This control of bubble growth rate reduces the possibility of abrupt rupturing of the bubble as it increases in diameter, and therefore allows the tubular member to be blown to a higher BUR. As a consequence of the increased BUR, the process of this invention will lead to balloons with thinner walls and hence lower profile balloon catheters, as compared to balloons of the same hoop strength or radial compliance made with today""s methods. A balloon made according to the method of the invention will have a higher hoop strength and lower radial compliance as compared to a balloon having the same wall thickness but made from conventional techniques in which internal pressure is delivered to the polymeric tubular member through a compressible fluid such as air or nitrogen. The method of the invention allows for high BURs without causing balloon defects such as microtears.
Additionally, a thin walled balloon of high hoop strength and low compliance is needed in the art. These and other advantages of the invention will become more apparent from the following detailed description and exemplary drawings.